The detection of fast neutrons is important for a number of applications, including nuclear safeguards, homeland security, and health physics. Charged radiation particles, such as alpha particles, typically create ion electron pairs as they slow down, providing a direct means for their detection. Being electrically neutral, neutrons do not interact with electrons like heavy charged particles. Instead, neutrons are indirectly detected after interacting with nuclei within a detector to produce a charged particle. For low energy neutrons (typically called thermal neutrons), there are a number of nuclear reactions that can produce energetic charged particles. Because the energy released (the Q value) during the reaction is much greater than the kinetic energy of the thermal neutron, the energy and direction of the resulting charged particles provide no information about the energy or direction of the incident neutron. For high energy neutrons (typically called fast neutrons) elastically scattering off nuclei, typically little or no energy is released, such that the energy and direction of the electrically charged recoil nuclei can be used to determine the direction and/or energy of the incident fast neutron.
Detectors based on the detection of recoil protons have been used for many years to detect and in some instances measure the energy and/or direction of fast neutrons. Fast neutrons elastically scatter off protons in the hydrogen atoms of the detector, resulting in recoil protons. (Approximately 99.99% of hydrogen nuclei consist of a single proton.) The energy and direction of the recoil protons depend on the energy and direction of the incident neutron and the scattering angle. Indeed, the collisions between neutrons and protons can be compared to billiard ball collisions. Because neutrons and protons have nearly equal masses, the neutron scattering angle ranges between 0° and 90°, and the energy of the recoil proton ranges between zero and the energy of the incident neutron. The most energetic recoil protons arc those scattered in the direction of the incident neutron, and the least energetic recoil protons are those scattered in a direction normal to the direction of the incident neutron.
Directional neutron detectors are typically designed either to determine the direction of incoming radiation, or to discriminate between sources of radiation coming from different directions. Thus, directional fast neutron detectors based on the detection of recoil protons can typically be classified into two broad categories: (1) those detectors that can determine the energy and direction of individual fast neutrons; and (2) those detectors that can preferentially discriminate fast neutrons incident from one direction from fast neutrons incident from other directions. The prior art includes various patents disclosing efforts to improve the science of directional neutron and/or radiation detectors. Patents thought to be the most relevant to the present invention are summarized as follows:
U.S. Pat. Nos. 5,029,262 and 5,036,202 to Schulte disclose bi-directional neutron detectors comprised of spaced hydrogenous layers interspersed with stacks of silicon detector layers. The signals produced by the silicon detectors are proportional to the energy loss in the detector layers, and these will vary as the proton loses more and more energy during its travel through the silicon detector layers. A pattern of energy loss is established through the stack of silicon layers which, in turn, is indicative as to whether the neutron producing the recoil proton has entered the detector from a front direction or a rear direction. This determination may be made without the necessity of measuring the track and total energy of a recoil proton as it passes through the detector.
U.S. Pat. No. 5,345,084 to Byrd discloses a segmented neutron detector consisting of a plurality of omni-directional radiation detectors arranged in a close packed symmetrical pattern. The output radiation counts from these detectors are arithmetically combined to provide the direction of a source of incident radiation. Indeed, output counts from paired detectors are subtracted to yield a vector direction toward the radiation source. The counts from all of the detectors can be combined to yield an output signal functionally related to the radiation source strength.
U.S. Pat. No. 5,880,469 to Miller discloses an apparatus and method for discriminating against neutrons coming from directions other than a preferred direction and also discriminating against gamma rays. Neutrons are detected through proton recoils in an array of optical scintillating fibers, and the optical fibers alternate between those which emit photons only in the lower portion of the electromagnetic spectrum and those which emit photons only in the higher portion of the electromagnetic spectrum. One end of the scintillating fibers is attached to one end of a light pipe. The other end of the light pipe is attached to two photomultiplier tubes (PMTs), parallel to each other. A signal processing unit registers a detected neutron if a signal is received from only one PMT and will register a background event if signals are received from both PMTs.
U.S. Pat. Nos. 6,495,837, 6,566,657 and 6,639,210 to Odom et al. disclose fast neutron detectors fabricated with alternating layers of hydrogenous, optically transparent, non scintillating material and scintillating material. The scintillating material is preferably zinc sulfide (ZnS), and the hydrogenous material is preferably plastic. Fast neutrons interact with the hydrogenous material generating recoil protons. The recoil protons enter the scintillating material resulting in scintillations. The detector is optically coupled to a PMT which generates electrical pulses proportional in amplitude to the intensity of the scintillations. Alternating layers of materials are dimensioned to optimize total efficiency of the detector.
U.S. Pat. No. 6,479,826 to Klann et al. discloses a device for detecting neutrons including a semi-insulated bulk semiconductor substrate having opposed polished surfaces. A blocking Schottky contact comprised of a series of metals such as Ti, Pt, Au, Ge, Pd, and Ni is formed on a first polished surface of the semiconductor substrate, while a low resistivity (“ohmic”) contact comprised of metals such as Au, Ge, and Ni is formed on a second, opposed polished surface of the substrate. Disposed on the Schottky contact is a neutron reactive film, or coating, for detecting neutrons. By varying the coating thickness and electrical settings, neutrons at specific energies can be detected.
U.S. Pat. No. 7,141,799 to Neal et al. discloses a detector system that combines a 6Li-loaded glass fiber scintillation thermal neutron detector with a fast scintillation detector in a single layered structure. Detection of thermal and fast neutrons and ionizing electromagnetic radiation is achieved in the unified detector structure. Fast neutrons, x-rays and gamma rays are detected in the fast scintillator. Thermal neutrons, x-rays and gamma rays are detected in the glass fiber scintillator.
While these prior art neutron detectors are useful for their intended purposes, a continuing need exists for an improved fast neutron detection system which is small in size, simple to operate, and more rugged than currently existing directional neutron detection systems, and also in which the efficiency and the sensitivity of neutron detection can be specified for a given application.